1. Field of the Invention
The present invention relates to a packaging plate (as used herein, the term “plate” shall include a “sheet”) for a magnetic circuit and a method for packaging a magnetic circuit using the same. More specifically, the present invention relates to the packaging of a permanent magnet-type magnetic circuit for a magnetic resonance imaging (MRI) system during its transportation and storage.
2. Description of Related Art
Magnet-type magnetic circuits comprising rare earth magnets has been mainly used for magnetic circuits comprising permanent magnets for magnetic resonance imaging systems. Such magnet-type magnetic circuits comprise magnets, yokes through which the magnetic flux from the magnets passes, and pole pieces that are ordinarily arranged on the magnet surface and made of a soft magnetic substance such as soft iron to generate a homogenous magnetic field in the gap between the magnets.
Referring to FIGS. 3 and 4, the following is an explanation of a conventional permanent magnet-type magnetic circuit. FIG. 3 is a schematic cross-sectional view of a permanent magnet-type magnetic circuit 101, taken from the front. FIG. 4 is a schematic cross-sectional view of the permanent magnet-type magnetic circuit 101, taken from the side. In FIGS. 3 and 4, the permanent magnet-type magnetic circuit 101 comprises a pair of permanent magnets 105, 106 which oppose each other across a gap and are magnetized in the direction of their thickness; yokes 102 to 104 which are arranged outside the gap between the permanent magnets, and which support the permanent magnets; and a pair of pole pieces 107, 108 each arranged on a gap side of the an opposing surface of each of the permanent magnets and having a peripheral projection in opposing orientation so that a magnetic field can be generated between the two pole pieces.
In such a magnet-type magnetic circuit, the yoke comprises upper and lower base plates (plate-shaped yokes) for having magnetic poles and one or two columns (column yoke) connecting these base plates, and the entire area except for the column can be regarded as the aperture portion. Such a magnet-type magnetic circuit has the advantage that its aperture is considerably wider than that of a superconductive solenoid coil-type magnetic circuit, and thus a patient will feel less occluded during his medical examination. On the other hand, it has the disadvantage that magnetic substances may easily penetrate into it from the outside. In general, rooms in which MRT units are installed are control areas and there are safeguards in place against the bringing of magnetic substances into such rooms. However, during transportation of the unit from factory to hospital, in the course of dislocation inside hospital or during a temporary storage etc., magnetic substances can be attracted to the magnetic circuit in unforeseeable ways, because the permanent magnet-type circuit always generates a magnetic field. Thus, there is the risk that both equipment and people may be endangered by magnetic substances sucked into the magnetic circuit. When the magnetic circuit is transported by a truck or the like, it has to be first placed in an iron container to reduce the leakage magnetic fields leaking out of the magnetic circuit, thus requiring troublesome handling.
The above-mentioned problem is caused by a leakage magnetic field escaping from the aperture of the magnetic circuit. For this reason, this problem can be lessened by packaging the magnetic circuit so as to reduce the magnetic field leaking from the aperture. It is effective to shield the magnetic circuit with a magnetic substance in order to reduce the leakage magnetic field. However, if a magnetic plate is brought too close to the aperture of the magnetic circuit, an attracting force will be generated due to permanent magnets installed in the magnetic circuit, so that the attaching of the magnetic plate becomes dangerous, and also its dismantling becomes difficult. For this reason, non-magnetic plates have been usually used for packaging magnetic circuits (see Japanese Patent No. 2965968 B). But because this alone cannot sufficiently reduce the leakage magnetic field, troublesome transport using an iron container has been required.